Salesforce Flows can be invoked from REST API. Flow API Name should be used in the end point /services/data/vxx.0/actions/custom/flow/Flow_API_Name to call it. 

Check the following example:

Sample Flow:

Flow Input Variable:

Flow Output Variable:

Fetch Records:

Workbench:
Endpoint:

/services/data/vxx.0/actions/custom/flow/Flow_API_Name

Request Body:

{
  "inputs": [
    {
      "InputVariableAPIName": "InputValue"
    }
  ]
}